1. Technical Field
The present invention relates to compressed air supply systems for installation on vehicles, and more particularly to a method of and apparatus for driving a compressor in such systems.
2. Description of the Problem
Conventional motor vehicle air brake systems are supplied with air from a compressed air supply system. The compressed air supply system includes an air compressor which has conventionally been driven by a mechanical linkage from the vehicle engine. The mechanical linkage is typically a belt, though a chain or gear connection is possible. The linkage is not readily released, and thus operates whenever the engine is running, even if the maximum desired air pressure in a system storage tank has been reached.
Various problems have been recognized with this arrangement. While the system is simple, it imposes a load on the engine which is parasitic (typically absorbing 2 horsepower) when the system storage tank is at maximum capacity because the pump/air compressor continues to run. The potential parasitic load on the engine is highest when it is running at high RPMs. The general issue of parasitic loads in motor vehicles has been recognized by the United States Department of Energy, which has sponsored a project called the “More Electric Truck” which contemplates replacing mechanical drives for accessories with electric motor(s) and thereby eliminating the source of much parasitic loss on vehicles. The direct mechanical linkage means that the compressor in such systems runs at a speed directly proportional to engine speed. The potential need to deliver high air flow to maintain system pressure or to support operation of systems utilizing air during periods of low engine speed may force the use of relatively high capacity, and therefore relatively heavy, components. Locations for installation of the compressor have been limited, as a practical matter, to the engine compartment, where space may be limited.
Beckman, et al., US 2006/0045751 taught an air compressor system for non-vehicle applications in which he proposed an embodiment using a variable speed electric motor to drive the air compressor. Beckman pointed out that prior art compressor systems utilizing electrical motors had the electrical motors operate at fixed speeds. As a result, the load on the motor increased as pressure in the compressed air reservoir increased and as a result more and more power was drawn until the pressure cutoff level was reached. Beckman's use of a variable speed electric motor allowed motor speed and pump speed to be varied. So doing allowed the motor “to operate at its maximum potential at all pressures”. Three operating modes were described, including a maximum mode in which the maximum allowable amperage was drawn, a “quiet mode” where the maximum amperage allowed is reduced, and an “optimum mode” where the motor was run at a minimum speed to maintain a target pressure. Broadly generalizing, Beckman's use of a variable speed motor allowed a user to select either an operating speed or a current limit. If a current limit was chosen operating speed of the motor was varied, as long as the motor was running, to keep the current drawn at the limit value.